Mechanistic study on the cross-coupling of alkynyl stannanes with aryl iodides catalyzed by η2-(dimethyl fumarate)palladium(0) complexes with iminophosphineligands

Abstract

The reactions of [Pd(η²-dmfu)(P–N)] [dmfu = dimethyl fumarate; P–N = 2-(PPh₂)C₆H₄-1-CHNR, R = C₆H₄OMe-4 (1a), CHMe₂ (2a)] and [Pd(η²-dmfu)(P–N)₂] with IC₆H₄CF₃-4, ISnBu₃ and PhCCSnBu₃ have been studied under pseudo-first-order conditions. The oxidative addition of IC₆H₄CF₃-4 yields [PdI(C₆H₄CF₃-4)(P–N)] (1b or 2b). No reaction takes place with PhCCSnBu₃ and also with ISnBu₃ in the presence of an excess of PhCCSnBu₃. In the presence of fumaronitrile (fn), 1b and 2b undergo transmetalation by PhCCSnBu₃ followed by fast reductive elimination to yield [Pd(η²-fn)(P–N)]. The same reaction sequence occurs for the system [PdI(C₆H₄CF₃-4)(P–N)]/P–N (1 ∶ 1 molar ratio) to give [Pd(η²-fn)(P–N)₂]. The palladium(0) complexes are active catalysts in the cross-coupling of PhCCSnBu₃ with aryl iodides ArI (Ar = C₆H₄CF₃-4, Ph). The catalytic efficiency depends on the complex: [Pd(η²-dmfu)(P–N)₂] > [Pd(η²-dmfu)(P–N)], and on the substituent R: C₆H₄OMe-4 > CHMe₂. The reactivity and spectroscopic data suggest a catalytic cycle involving initial oxidative addition of ArI to a palladium(0) species, followed by transmetalation of the product and by fast reductive elimination to regenerate the starting palladium(0) compound. For [Pd(η²-dmfu)(P–N)] as catalyst, the oxidative addition is the rate-determining step, while for [Pd(η²-dmfu)(P–N)₂] the oxidative addition and the transmetalation steps occur at comparable rate.